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When the axes of the first gear (i.e. first driver) and the last gear (i.e. last driven or follower) are co-axial, then the gear train is known as reverted gear train as shown in Fig. 3. Fig. 3. Reverted gear We see that gear 1 (i.e. first driver) drives the gear 2 (i.e. first driven or follower) in the opposite direction.  Since the gears 2 and 3 are mounted on the same shaft, therefore they form a compound gear and the gear 3 will rotate in the same direction as that of gear 2.  The gear 3 (which is now the second driver) drives the gear 4 (i.e. the last driven or follower) in the same direction as that of gear 1.  Thus we see that in a reverted gear train, the motion of the first gear and the last gear is  al ike. Let  T 1  = Number of teeth on gear 1, r 1  = Pitch circle radius of gear 1, and N 1  = Speed of gear 1 in r.p.m. Similarly, T 2 , T 3 , T 4  = Number of teeth on respective gears, r 2 , r 3 , r 4  = Pitch circle radii of respective gears, and N 2 , N 3 , N 4  = Speed of r

When there is more than one gear on a shaft, as shown in Fig.2, it is called a  compound train of gear. We have seen in the previous section that the  idle gears,  in a simple train of gears  do not affect the speed ratio  of the system. But these gears are useful in bridging over the space between the driver and the driven.  But whenever the distance between the driver and the driven or follower  has to be bridged  over by intermediate gears and  at the same time a great ( or much less ) speed ratio is required , then the advantage of intermediate gears is intensified by providing compound gears on intermediate shafts.  In this case,  each intermediate shaft has two gears rigidly fixed  to it so that they may have the same speed. One of these two gears meshes with the driver and the other with the driven or follower attached to the next shaft as shown in Fig. 2. Fig. 2 Compounded gear train In a compound train of gears, as shown in Fig. 2, the gear 1 is the driving gear mounted on sha

When there is only one gear on each shaft, as shown in Fig. 1, it is known as a simple gear train. The gears are represented by their pitch circles. When the distance between the two shafts is small, the two gears 1 and 2 are made to mesh with each other to transmit motion from one shaft to the other, as shown in Fig. 1 (a). Since the gear 1 drives the gear 2, therefore gear 1 is called the driver and the gear 2 is called the driven or follower. It may be noted that the motion of the driven gear is opposite to the motion of driving gear. Fig. 1. Simple gear train Let  N 1  = Speed of gear 1(or driver) in r.p.m., N 2  = Speed of gear 2 (or driven or follower) in r.p.m., T 1  = Number of teeth on gear 1, and T 2  = Number of teeth on gear 2. Since the  speed ratio (or velocity ratio) of the gear train  is the ratio of the speed of the driver to the speed of the driven or follower and ratio of speeds of any pair of gears in the mesh is the inverse of their number of teeth, therefore It ma

Gears can be classified: According to the relative position of their axes According to the type of contact between the surface of the gear Figure 1: Types of gears Parallel shaft Depending upon the teeth of the equivalent cylinder, i.e., straight or helix  We have following parallel shaft gears Spur gears: straight teeth parallel to the axes of the wheel Helical gears: teeth are curved and inclined to the shaft axis. Two mating gears have the same helix angle but have teeth of the opposite hand Herringbone gears: A double-helical gear is equivalent to a pair of helical gears attached together, one has a right-hand helix and other a left-hand helix Figure 2: Spur gear Figure 3: Helical gear Figure 4: Herringbone gear Intersecting gear Two non-parallel or intersecting shafts can be connected by means of bevel gears.  Straight bevel gears: teeth are straight, radial to the point of intersection of the shaft axes, and vary in cross-section throughout their length. Used to connect shaft